Activation of the multifunctional Ca2+/calmodulin-dependent protein kinase by autophosphorylation: ATP modulates production of an autonomous enzyme

Ruxolitinib as a CaMKII inhibitor for treatment of cardiac arrhythmias: Applications and prospects

Recent studies have highlighted the critical role of calcium/calmodulin-dependent protein kinase II (CaMKII) overactivation in the pathogenesis of various cardiac arrhythmias. Ruxolitinib, a Janus kinase inhibitor widely used for the treatment of myelofibrosis and acute graft-vs-host disease, has expanded its research horizons to include its potential as a CaMKII inhibitor in the treatment of cardiac arrhythmias. This article reviews the basic pharmacologic properties of ruxolitinib and delves into the role of CaMKII in cardiac arrhythmias, including its structural fundamentals, activation mechanisms, and association with arrhythmic conditions. Furthermore, the current state of CaMKII inhibitor research is discussed, with a special focus on the advances and clinical potential of ruxolitinib in this field. Studies indicate that ruxolitinib effectively inhibits CaMKII activity and has therapeutic potential against cardiac arrhythmias in animal models and at the cellular level. In addition, we address the critical issues that need to be resolved before the clinical application of ruxolitinib in arrhythmia treatment, including dosage concerns, long-term inhibitory effects, potential impacts on the nervous system, and efficacy across different types of arrhythmias. Future research directions involve further exploration of the clinical application potential of ruxolitinib, particularly in diseases such as heart failure, hypertrophic cardiomyopathy, dilated cardiomyopathy, and ischemic arrhythmias. In summary, the efficacy, low toxicity, and safety profile of ruxolitinib as a CaMKII inhibitor in the treatment of cardiac arrhythmias suggest a promising future for its development as a therapeutic drug in this domain.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

CaMKII autophosphorylation can occur between holoenzymes without subunit exchange

The dodecameric protein kinase CaMKII is expressed throughout the body. The alpha isoform is responsible for synaptic plasticity and participates in memory through its phosphorylation of synaptic proteins. Its elaborate subunit organization and propensity for autophosphorylation allow it to preserve neuronal plasticity across space and time. The prevailing hypothesis for the spread of CaMKII activity, involving shuffling of subunits between activated and naive holoenzymes, is broadly termed subunit exchange. In contrast to the expectations of previous work, we found little evidence for subunit exchange upon activation, and no effect of restraining subunits to their parent holoenzymes. Rather, mass photometry, crosslinking mass spectrometry, single molecule TIRF microscopy and biochemical assays identify inter-holoenzyme phosphorylation (IHP) as the mechanism for spreading phosphorylation. The transient, activity-dependent formation of groups of holoenzymes is well suited to the speed of neuronal activity. Our results place fundamental limits on the activation mechanism of this kinase.

CaMKII as a Therapeutic Target in Cardiovascular Disease

CaMKII (the multifunctional Ca2+ and calmodulin-dependent protein kinase II) is a highly validated signal for promoting a variety of common diseases, particularly in the cardiovascular system. Despite substantial amounts of convincing preclinical data, CaMKII inhibitors have yet to emerge in clinical practice. Therapeutic inhibition is challenged by the diversity of CaMKII isoforms and splice variants and by physiological CaMKII activity that contributes to learning and memory. Thus, uncoupling the harmful and beneficial aspects of CaMKII will be paramount to developing effective therapies. In the last decade, several targeting strategies have emerged, including small molecules, peptides, and nucleotides, which hold promise in discriminating pathological from physiological CaMKII activity. Here we review the cellular and molecular biology of CaMKII, discuss its role in physiological and pathological signaling, and consider new findings and approaches for developing CaMKII therapeutics.

CaMKII oxidation is a critical performance/disease trade-off acquired at the dawn of vertebrate evolution

Antagonistic pleiotropy is a foundational theory that predicts aging-related diseases are the result of evolved genetic traits conferring advantages early in life. Here we examine CaMKII, a pluripotent signaling molecule that contributes to common aging-related diseases, and find that its activation by reactive oxygen species (ROS) was acquired more than half-a-billion years ago along the vertebrate stem lineage. Functional experiments using genetically engineered mice and flies reveal ancestral vertebrates were poised to benefit from the union of ROS and CaMKII, which conferred physiological advantage by allowing ROS to increase intracellular Ca²⁺ and activate transcriptional programs important for exercise and immunity. Enhanced sensitivity to the adverse effects of ROS in diseases and aging is thus a trade-off for positive traits that facilitated the early and continued evolutionary success of vertebrates.

Natural selection may favor traits underlying aging-related diseases if they benefit the young. Wang et al. find that oxidative activation of CaMKII provides physiological benefits critical to the initial and continued success of vertebrates but at the cost of disease, frailty, and shortened lifespan.

CaMKII holoenzyme mechanisms that govern the LTP versus LTD decision

Higher brain functions are thought to require synaptic frequency decoding that can lead to long-term potentiation (LTP) or depression (LTD). We show that the LTP versus LTD decision is determined by complex cross-regulation of T286 and T305/306 autophosphorylation within the 12meric CaMKII holoenzyme, which enabled molecular computation of stimulus frequency, amplitude, and duration. Both LTP and LTD require T286 phosphorylation, but T305/306 phosphorylation selectively promoted LTD. In response to excitatory LTP versus LTD stimuli, the differential T305/306 phosphorylation directed CaMKII movement to either excitatory or inhibitory synapses, thereby coordinating plasticity at both synapse types. Fast T305/306 phosphorylation required prior T286 phosphorylation and then curbed CaMKII activity by two mechanisms: (i) a cis-subunit reaction reduced both Ca²⁺ stimulation and autonomous activity and (ii) a trans-subunit reaction enabled complete activity shutdown and feed-forward inhibition of further T286 phosphorylation. These are fundamental additions to the long-studied CaMKII regulation and function in neuronal plasticity.

Role of calcium/calmodulin-dependent kinase 2 in neurodevelopmental disorders

Neurodevelopmental disorders are a complex and heterogeneous group of neurological disorders characterized by their early-onset and estimated to affect more than 3% of children worldwide. The rapid advancement of sequencing technologies in the past years allowed the identification of hundreds of variants in several different genes causing neurodevelopmental disorders. Between those, new variants in the Calcium/calmodulin dependent protein kinase II (CAMK2) genes were recently linked to intellectual disability. Despite many years of research on CAMK2, this proves for the first time that this well-known and highly conserved molecule plays an important role in the human brain. In this review, we give an overview of the identified CAMK2 variants, and we speculate on potential mechanisms through which dysfunctions in CAMK2 result in neurodevelopmental disorders. Additionally, we discuss how the identification of CAMK2 variants might result in new exciting discoveries regarding the function of CAMK2 in the human brain.

MICAL1 constrains cardiac stress responses and protects against disease by oxidizing CaMKII

Oxidant stress can contribute to health and disease. Here we show that invertebrates and vertebrates share a common stereospecific redox pathway that protects against pathological responses to stress, at the cost of reduced physiological performance, by constraining Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity. MICAL1, a methionine monooxygenase thought to exclusively target actin, and MSRB, a methionine reductase, control the stereospecific redox status of M308, a highly conserved residue in the calmodulin-binding (CaM-binding) domain of CaMKII. Oxidized or mutant M308 (M308V) decreased CaM binding and CaMKII activity, while absence of MICAL1 in mice caused cardiac arrhythmias and premature death due to CaMKII hyperactivation. Mimicking the effects of M308 oxidation decreased fight-or-flight responses in mice, strikingly impaired heart function in Drosophila melanogaster, and caused disease protection in human induced pluripotent stem cell-derived cardiomyocytes with catecholaminergic polymorphic ventricular tachycardia, a CaMKII-sensitive genetic arrhythmia syndrome. Our studies identify a stereospecific redox pathway that regulates cardiac physiological and pathological responses to stress across species.

Structural Insights into the Regulation of Ca2+/Calmodulin-Dependent Protein Kinase II (CaMKII)

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a highly conserved serine/threonine kinase that is ubiquitously expressed throughout the human body. Specialized isoforms of CaMKII play key roles in neuronal and cardiac signaling. The distinctive holoenzyme architecture of CaMKII, with 12-14 kinase domains attached by flexible linkers to a central hub, poses formidable challenges for structural characterization. Nevertheless, progress in determining the structural mechanisms underlying CaMKII functions has come from studying the kinase domain and the hub separately, as well as from a recent electron microscopic investigation of the intact holoenzyme. In this review, we discuss our current understanding of the structure of CaMKII. We also discuss the intriguing finding that the CaMKII holoenzyme can undergo activation-triggered subunit exchange, a process that has implications for the potentiation and perpetuation of CaMKII activity.

Flexible linkers in CaMKII control the balance between activating and inhibitory autophosphorylation

The many variants of human Ca2+/calmodulin-dependent protein kinase II (CaMKII) differ in the lengths and sequences of disordered linkers connecting the kinase domains to the oligomeric hubs of the holoenzyme. CaMKII activity depends on the balance between activating and inhibitory autophosphorylation (on Thr 286 and Thr 305/306, respectively, in the human α isoform). Variation in the linkers could alter transphosphorylation rates within a holoenzyme and the balance of autophosphorylation outcomes. We show, using mammalian cell expression and a single-molecule assay, that the balance of autophosphorylation is flipped between CaMKII variants with longer and shorter linkers. For the principal isoforms in the brain, CaMKII-α, with a ~30 residue linker, readily acquires activating autophosphorylation, while CaMKII-β, with a ~200 residue linker, is biased towards inhibitory autophosphorylation. Our results show how the responsiveness of CaMKII holoenzymes to calcium signals can be tuned by varying the relative levels of isoforms with long and short linkers.

The cardiac CaMKII-Nav1.5 relationship: From physiology to pathology

The SCN5A gene encodes Nav1.5, which, as the cardiac voltage-gated Na+ channel's pore-forming α subunit, is crucial for the initiation and propagation of atrial and ventricular action potentials. The arrhythmogenic propensity of inherited SCN5A mutations implicates the Na+ channel in determining cardiomyocyte excitability under normal conditions. Cytosolic kinases have long been known to alter the kinetic profile of Nav1.5 inactivation via phosphorylation of specific residues. Recent substantiation of both the role of calmodulin-dependent kinase II (CaMKII) in modulating the properties of the Nav1.5 inactivation gate and the significant rise in oxidation-dependent autonomous CaMKII activity in structural heart disease has raised the possibility of a novel pathway for acquired arrhythmias - the CaMKII-Nav1.5 relationship. The aim of this review is to: (1) outline the relationship's translation from physiological adaptation to pathological vicious circle; and (2) discuss the relative merits of each of its components as pharmacological targets.

CaM Kinase: Still Inspiring at 40

The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) was touted as a memory molecule, even before its involvement in long-term potentiation (LTP) was shown. The enzyme has not disappointed, with subsequent demonstrations of remarkable structural and regulatory properties. Its neuronal functions now extend to long-term depression (LTD), and last year saw the first direct evidence for memory storage by CaMKII. Although CaMKII may have taken the spotlight, it is a member of a large family of diverse and interesting CaM kinases. Our aim is to place CaMKII in context of the other CaM kinases and then review certain aspects of this kinase that are of current interest.

eNOS-NO-induced small blood vessel relaxation requires EHD2-dependent caveolae stabilization

Endothelial nitric oxide synthase (eNOS)-related vessel relaxation is a highly coordinated process that regulates blood flow and pressure and is dependent on caveolae. Here, we investigated the role of caveolar plasma membrane stabilization by the dynamin-related ATPase EHD2 on eNOS-nitric oxide (NO)-dependent vessel relaxation. Loss of EHD2 in small arteries led to increased numbers of caveolae that were detached from the plasma membrane. Concomitantly, impaired relaxation of mesenteric arteries and reduced running wheel activity were observed in EHD2 knockout mice. EHD2 deletion or knockdown led to decreased production of nitric oxide (NO) although eNOS expression levels were not changed. Super-resolution imaging revealed that eNOS was redistributed from the plasma membrane to internalized detached caveolae in EHD2-lacking tissue or cells. Following an ATP stimulus, reduced cytosolic Ca2+ peaks were recorded in human umbilical vein endothelial cells (HUVECs) lacking EHD2. Our data suggest that EHD2-controlled caveolar dynamics orchestrates the activity and regulation of eNOS/NO and Ca2+ channel localization at the plasma membrane.

Loss of MCU prevents mitochondrial fusion in G1-S phase and blocks cell cycle progression and proliferation

The role of the mitochondrial Ca²⁺ uniporter (MCU) in physiologic cell proliferation remains to be defined. Here, we demonstrated that the MCU was required to match mitochondrial function to metabolic demands during the cell cycle. During the G₁-S transition (the cycle phase with the highest mitochondrial ATP output), mitochondrial fusion, oxygen consumption, and Ca²⁺ uptake increased in wild-type cells but not in cells lacking MCU. In proliferating wild-type control cells, the addition of the growth factors promoted the activation of the Ca²⁺/calmodulin-dependent kinase II (CaMKII) and the phosphorylation of the mitochondrial fission factor Drp1 at Ser⁶¹⁶ The lack of the MCU was associated with baseline activation of CaMKII, mitochondrial fragmentation due to increased Drp1 phosphorylation, and impaired mitochondrial respiration and glycolysis. The mitochondrial fission/fusion ratio and proliferation in MCU-deficient cells recovered after MCU restoration or inhibition of mitochondrial fragmentation or of CaMKII in the cytosol. Our data highlight a key function for the MCU in mitochondrial adaptation to the metabolic demands during cell cycle progression. Cytosolic CaMKII and the MCU participate in a regulatory circuit, whereby mitochondrial Ca²⁺ uptake affects cell proliferation through Drp1.

Impact of Genetic Variation on Human CaMKK2 Regulation by Ca2+-Calmodulin and Multisite Phosphorylation

The Ca²⁺-calmodulin dependent protein kinase kinase-2 (CaMKK2) is a key regulator of neuronal function and whole-body energy metabolism. Elevated CaMKK2 activity is strongly associated with prostate and hepatic cancers, whereas reduced CaMKK2 activity has been linked to schizophrenia and bipolar disease in humans. Here we report the functional effects of nine rare-variant point mutations that were detected in large-scale human genetic studies and cancer tissues, all of which occur close to two regulatory phosphorylation sites and the catalytic site on human CaMKK2. Four mutations (G87R, R139W, R142W and E268K) cause a marked decrease in Ca²⁺-independent autonomous activity, however S137L and P138S mutants displayed increased autonomous and Ca²⁺-CaM stimulated activities. Furthermore, the G87R mutant is defective in Thr85-autophosphorylation dependent autonomous activity, whereas the A329T mutation rendered CaMKK2 virtually insensitive to Ca²⁺-CaM stimulation. The G87R and R139W mutants behave as dominant-negative inhibitors of CaMKK2 signaling in cells as they block phosphorylation of the downstream substrate AMP-activated protein kinase (AMPK) in response to ionomycin. Our study provides insight into functionally disruptive, rare-variant mutations in human CaMKK2, which have the potential to influence risk and burden of disease associated with aberrant CaMKK2 activity in human populations carrying these variants.

Global sensitivity analysis of a model related to memory formation in synapses: Model reduction based on epistemic parameter uncertainties and related issues

We investigate the epistemic uncertainties of parameters of a mathematical model that describes the dynamics of CaMKII-NMDAR complex related to memory formation in synapses using global sensitivity analysis (GSA). The model, which was published in this journal, is nonlinear and complex with Ca²⁺ patterns with different level of frequencies as inputs. We explore the effects of parameter on the key outputs of the model to discover the most sensitive ones using GSA and partial ranking correlation coefficient (PRCC) and to understand why they are sensitive and others are not based on the biology of the problem. We also extend the model to add presynaptic neurotransmitter vesicles release to have action potentials as inputs of different frequencies. We perform GSA on this extended model to show that the parameter sensitivities are different for the extended model as shown by PRCC landscapes. Based on the results of GSA and PRCC, we reduce the original model to a less complex model taking the most important biological processes into account. We validate the reduced model against the outputs of the original model. We show that the parameter sensitivities are dependent on the inputs and GSA would make us understand the sensitivities and the importance of the parameters. A thorough phenomenological understanding of the relationships involved is essential to interpret the results of GSA and hence for the possible model reduction.

Vascular CaMKII: heart and brain in your arteries

First characterized in neuronal tissues, the multifunctional calcium/calmodulin-dependent protein kinase II (CaMKII) is a key signaling component in several mammalian biological systems. Its unique capacity to integrate various Ca(2+) signals into different specific outcomes is a precious asset to excitable and nonexcitable cells. Numerous studies have reported roles and mechanisms involving CaMKII in brain and heart tissues. However, corresponding functions in vascular cell types (endothelium and vascular smooth muscle cells) remained largely unexplored until recently. Investigation of the intracellular Ca(2+) dynamics, their impact on vascular cell function, the regulatory processes involved and more recently the spatially restricted oscillatory Ca(2+) signals and microdomains triggered significant interest towards proteins like CaMKII. Heteromultimerization of CaMKII isoforms (four isoforms and several splice variants) expands this kinase's peculiar capacity to decipher Ca(2+) signals and initiate specific signaling processes, and thus controlling cellular functions. The physiological functions that rely on CaMKII are unsurprisingly diverse, ranging from regulating contractile state and cellular proliferation to Ca(2+) homeostasis and cellular permeability. This review will focus on emerging evidence of CaMKII as an essential component of the vascular system, with a focus on the kinase isoform/splice variants and cellular system studied.

The role of CaMKII in diabetic heart dysfunction

Diabetes mellitus (DM) is an increasing epidemic that places a significant burden on health services worldwide. The incidence of heart failure (HF) is significantly higher in diabetic patients compared to non-diabetic patients. One underlying mechanism proposed for the link between DM and HF is activation of calmodulin-dependent protein kinase (CaMKIIδ). CaMKIIδ mediates ion channel function and Ca(2+) handling during excitation-contraction and excitation-transcription coupling in the myocardium. CaMKIIδ activity is up-regulated in the myocardium of diabetic patients and mouse models of diabetes, where it promotes pathological signaling that includes hypertrophy, fibrosis and apoptosis. Pharmacological inhibition and knockout models of CaMKIIδ have shown some promise of a potential therapeutic benefit of CaMKIIδ inhibition, with protection against cardiac hypertrophy and apoptosis reported. This review will highlight the pathological role of CaMKIIδ in diabetes and discuss CaMKIIδ as a therapeutic target in DM, and also the effects of exercise on CaMKIIδ.

Neural plasticity and behavior - sixty years of conceptual advances

This brief review summarizes 60 years of conceptual advances that have demonstrated a role for active changes in neuronal connectivity as a controller of behavior and behavioral change. Seminal studies in the first phase of the six-decade span of this review firmly established the cellular basis of behavior - a concept that we take for granted now, but which was an open question at the time. Hebbian plasticity, including long-term potentiation and long-term depression, was then discovered as being important for local circuit refinement in the context of memory formation and behavioral change and stabilization in the mammalian central nervous system. Direct demonstration of plasticity of neuronal circuit function in vivo, for example, hippocampal neurons forming place cell firing patterns, extended this concept. However, additional neurophysiologic and computational studies demonstrated that circuit development and stabilization additionally relies on non-Hebbian, homoeostatic, forms of plasticity, such as synaptic scaling and control of membrane intrinsic properties. Activity-dependent neurodevelopment was found to be associated with cell-wide adjustments in post-synaptic receptor density, and found to occur in conjunction with synaptic pruning. Pioneering cellular neurophysiologic studies demonstrated the critical roles of transmembrane signal transduction, NMDA receptor regulation, regulation of neural membrane biophysical properties, and back-propagating action potential in critical time-dependent coincidence detection in behavior-modifying circuits. Concerning the molecular mechanisms underlying these processes, regulation of gene transcription was found to serve as a bridge between experience and behavioral change, closing the 'nature versus nurture' divide. Both active DNA (de)methylation and regulation of chromatin structure have been validated as crucial regulators of gene transcription during learning. The discovery of protein synthesis dependence on the acquisition of behavioral change was an influential discovery in the neurochemistry of behavioral modification. Higher order cognitive functions such as decision making and spatial and language learning were also discovered to hinge on neural plasticity mechanisms. The role of disruption of these processes in intellectual disabilities, memory disorders, and drug addiction has recently been clarified based on modern genetic techniques, including in the human. The area of neural plasticity and behavior has seen tremendous advances over the last six decades, with many of those advances being specifically in the neurochemistry domain. This review provides an overview of the progress in the area of neuroplasticity and behavior over the life-span of the Journal of Neurochemistry. To organize the broad literature base, the review collates progress into fifteen broad categories identified as 'conceptual advances', as viewed by the author. The fifteen areas are delineated in the figure above. This article is part of the 60th Anniversary special issue.

Molecular mechanism of activation-triggered subunit exchange in Ca(2+)/calmodulin-dependent protein kinase II

Activation triggers the exchange of subunits in Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), an oligomeric enzyme that is critical for learning, memory, and cardiac function. The mechanism by which subunit exchange occurs remains elusive. We show that the human CaMKII holoenzyme exists in dodecameric and tetradecameric forms, and that the calmodulin (CaM)-binding element of CaMKII can bind to the hub of the holoenzyme and destabilize it to release dimers. The structures of CaMKII from two distantly diverged organisms suggest that the CaM-binding element of activated CaMKII acts as a wedge by docking at intersubunit interfaces in the hub. This converts the hub into a spiral form that can release or gain CaMKII dimers. Our data reveal a three-way competition for the CaM-binding element, whereby phosphorylation biases it towards the hub interface, away from the kinase domain and calmodulin, thus unlocking the ability of activated CaMKII holoenzymes to exchange dimers with unactivated ones.

DOI:http://dx.doi.org/10.7554/eLife.13405.001

How does memory outlast the lifetime of the molecules that encode it? One enzyme that is found in neurons and has been suggested to help long-term memories to form is called CaMKII. Each CaMKII assembly is typically composed of 12 to 14 protein subunits associated in a ring and can exist in either an “unactivated” or “activated” state. In 2014, researchers showed that CaMKII assemblies can exchange subunits with each other. Importantly, an active CaMKII can mix with an unactivated CaMKII and share its activation state. CaMKII may use this mechanism to spread information to the next generation of proteins – thereby allowing activation to outlast the lifespan of the initially activated proteins. However the molecular mechanism that underlies this process was not clear.

Now, Bhattacharyya et al. – including some of the researchers involved in the 2014 work – address two questions about this mechanism. How do subunits exchange between CaMKII assemblies? And how does the activation of CaMKII initiate subunit exchange?

A closed-ring hub ties the subunits of CaMKII together, similar to the organization of the segments in an orange. To undergo subunit exchange, the hub must open up to release and accept subunits. Bhattacharyya et al. have now uncovered an intrinsic flexibility in the hub that is triggered by a short peptide segment in CaMKII. This segment, which is exposed in activated CaMKII but not in the unactivated form, can crack open the hub ring by binding between the hub subunits, like a finger separating the segments of an orange. This allows the hub to flex and expand, and once open, the hub’s flexibility allows room for subunits to be released or accepted.

Although this subunit exchange mechanism could be a powerful means for spreading the activated state throughout signaling pathways, the biological relevance of this phenomenon has not been clarified. However, the mechanistic framework provided by Bhattacharyya et al. may allow new experiments to be performed that test the consequences of subunit exchange in live cells and organisms. It could also enable investigations into the importance of subunit exchange in long-term memory.

DOI:http://dx.doi.org/10.7554/eLife.13405.002

Atrial remodelling in atrial fibrillation: CaMKII as a nodal proarrhythmic signal

CaMKII is a serine-threonine protein kinase that is abundant in myocardium. Emergent evidence suggests that CaMKII may play an important role in promoting atrial fibrillation (AF) by targeting a diverse array of proteins involved in membrane excitability, cell survival, calcium homeostasis, matrix remodelling, inflammation, and metabolism. Furthermore, CaMKII inhibition appears to protect against AF in animal models and correct proarrhythmic, defective intracellular Ca(2+) homeostasis in fibrillating human atrial cells. This review considers current concepts and evidence from animal and human studies on the role of CaMKII in AF.

Modelling the dynamics of CaMKII-NMDAR complex related to memory formation in synapses: the possible roles of threonine 286 autophosphorylation of CaMKII in long term potentiation

A synaptic protein, Ca(2+)/Calmodulin dependent protein kinase II (CaMKII), has complex state transitions and facilitates the emergence of long term potentiation (LTP), which is highly correlated to memory formation. Two of the state transitions are critical for LTP: (1) threonine 286 autophosphorylation of CaMKII; and (2) binding to N-methyl-d-aspartate receptor (NMDAR) in the postsynaptic density (PSD) to form CaMKII-NMDAR complex. Both of these state transitions retain the activity of CaMKII when the induction signal disappears which is very important for the long-lasting characteristics of LTP. However, the possible relationships between the state transitions in the emergence of LTP are not well understood. We develop a mathematical model of the formation of CaMKII-NMDAR complex with the full state transitions of CaMKII, including the autophosphorylation, based on ordinary differential equations. In addition, we formulate a probabilistic framework for the binding between CaMKII and NMDAR. The model gives accurate predictions of the behaviours of CaMKII in comparisons to the experimental observations. Using the model, we show that: (1) the formation of CaMKII-NMDAR complex is dependent not only on the binding affinity between CaMKII and NMDAR, but also on the translocation of CaMKII into PSD; and (2) the autophosphorylation is not a requirement for the formation of CaMKII-NMDAR complex, but is important for the rapid formation of CaMKII-NMDAR complex during LTP.

CaMKII isoforms differ in their specific requirements for regulation by nitric oxide

The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) mediates physiological and pathological functions by its Ca(2+)-independent autonomous activity. Two novel mechanisms for generating CaMKII autonomy include oxidation and S-nitrosylation, the latter requiring both Cys280 and Cys289 amino acid residues in the brain-specific isoform CaMKIIα. Even though the other CaMKII isoforms have a different amino acid in the position homologous to Cys280, we show here that nitric oxide (NO)-signaling generated autonomy also for the CaMKIIβ isoform. Furthermore, although oxidation of the Met280/281 residues is sufficient to generate autonomy for most CaMKII isoforms, oxidation-induced autonomy was also prevented by a Cys289-mutation in the CaMKIIα isoform. Thus, all CaMKII isoforms can be regulated by physiological NO-signaling, but CaMKIIα regulation by oxidation and S-nitrosylation is more stringent.

Excitotoxic glutamate insults block autophagic flux in hippocampal neurons

Excitotoxic insults such as cerebral ischemia are thought to enhance neuronal autophagy, which is then thought to promote neuronal cell death. Excitotoxic insults indeed increase autophagy markers. Notably, however, autophagy markers can be increased either by autophagy induction (as this enhances their production) or by late-stage autophagy inhibition (as this prevents their degradation during autophagic flux). By comparing each condition with and without protease inhibitors that prevent autophagic degradation of the autophagy markers, the results of this study show that excitotoxic glutamate increases autophagy markers by a late-stage block of autophagy. Initially, this study set out to test if the CaMKII inhibitor tatCN21 mediates its post-insult neuroprotection by regulating autophagy. While tatCN21 partially inhibited basal autophagy in hippocampal neurons, it had no effects on the already blocked autophagy after excitotoxic glutamate insults, indicating that autophagy inhibition is not its neuroprotective mechanism. Additionally, while the autophagy inhibitor chloroquine had no effect, significant neuroprotection was seen instead with two drugs that enhance autophagy induction by different mechanisms, rapamycin (mTOR-dependent) and trehalose (mTOR-independent). This suggests that therapeutic approaches should seek to enhance rather than inhibit autophagy, not only in neurodegenerative diseases (where such approach is widely accepted) but also after acute excitotoxic insults. Together, these findings significantly reshape the current view on the mutual cross-regulation of autophagy and excitotoxicity.

Nitric oxide induces Ca2+-independent activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII)

Both signaling by nitric oxide (NO) and by the Ca(2+)/calmodulin (CaM)-dependent protein kinase II α isoform (CaMKIIα) are implicated in two opposing forms of synaptic plasticity underlying learning and memory, as well as in excitotoxic/ischemic neuronal cell death. For CaMKIIα, these functions specifically involve also Ca(2+)-independent autonomous activity, traditionally generated by Thr-286 autophosphorylation. Here, we demonstrate that NO-induced S-nitrosylation of CaMKIIα also directly generated autonomous activity, and that CaMKII inhibition protected from NO-induced neuronal cell death. NO induced S-nitrosylation at Cys-280/289, and mutation of either site abolished autonomy, indicating that simultaneous nitrosylation at both sites was required. Additionally, autonomy was generated only when Ca(2+)/CaM was present during NO exposure. Thus, generation of this form of CaMKIIα autonomy requires simultaneous signaling by NO and Ca(2+). Nitrosylation also significantly reduced subsequent CaMKIIα autophosphorylation specifically at Thr-286, but not at Thr-305. A previously described reduction of CaMKII activity by S-nitrosylation at Cys-6 was also observed here, but only after prolonged (>5 min) exposure to NO donors. These results demonstrate a novel regulation of CaMKII by another second messenger system and indicate its involvement in excitotoxic neuronal cell death.

Autonomous CaMKII requires further stimulation by Ca2+/calmodulin for enhancing synaptic strength

A hallmark feature of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is generation of autonomous (Ca(2+)-independent) activity by T286 autophosphorylation. Biochemical studies have shown that "autonomous" CaMKII is ∼5-fold further stimulated by Ca(2+)/CaM, but demonstration of a physiological function for such regulation within cells has remained elusive. In this study, CaMKII-induced enhancement of synaptic strength in rat hippocampal neurons required both autonomous activity and further stimulation. Synaptic strength was decreased by CaMKIIα knockdown and rescued by reexpression, but not by mutants impaired for autonomy (T286A) or binding to NMDA-type glutamate receptor subunit 2B (GluN2B; formerly NR2B; I205K). Full rescue was seen with constitutively autonomous mutants (T286D), but only if they could be further stimulated (additional T305/306A mutation), and not with two other mutations that additionally impair Ca(2+)/CaM binding. Compared to rescue with wild-type CaMKII, the CaM-binding-impaired mutants even had reduced synaptic strength. One of these mutants (T305/306D) mimicked an inhibitory autophosphorylation of CaMKII, whereas the other one (Δstim) abolished CaM binding without introducing charged residues. Inhibitory T305/306 autophosphorylation also reduced GluN2B binding, but this effect was independent of reduced Ca(2+)/CaM binding and was not mimicked by T305/306D mutation. Thus, even autonomous CaMKII activity must be further stimulated by Ca(2+)/CaM for enhancement of synaptic strength.

New antiarrhythmic targets to control intracellular calcium handling

Sudden cardiac death due to ventricular arrhythmias is a major problem. Drug therapies to prevent SCD do not provide satisfying results, leading to the demand for new antiarrhythmic strategies. New targets include Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII), the Na/Ca exchanger (NCX), the Ryanodine receptor (RyR, and its associated protein FKBP12.6 (Calstabin)) and the late component of the sodium current (I_Na-Late), all related to intracellular calcium (Ca²⁺) handling. In this review, drugs interfering with these targets (SEA-0400, K201, KN-93, W7, ranolazine, sophocarpine, and GS-967) are evaluated and their future as clinical compounds is considered. These new targets prove to be interesting; however more insight into long-term drug effects is necessary before clinical applicability becomes reality.

Mechanochemotransduction during cardiomyocyte contraction is mediated by localized nitric oxide signaling

Cardiomyocytes contract against a mechanical load during each heartbeat, and excessive mechanical stress leads to heart diseases. Using a cell-in-gel system that imposes an afterload during cardiomyocyte contraction, we found that nitric oxide synthase (NOS) was involved in transducing mechanical load to alter Ca(2+) dynamics. In mouse ventricular myocytes, afterload increased the systolic Ca(2+) transient, which enhanced contractility to counter mechanical load but also caused spontaneous Ca(2+) sparks during diastole that could be arrhythmogenic. The increases in the Ca(2+) transient and sparks were attributable to increased ryanodine receptor (RyR) sensitivity because the amount of Ca2(+) in the sarcoplasmic reticulum load was unchanged. Either pharmacological inhibition or genetic deletion of nNOS (or NOS1), but not of eNOS (or NOS3), prevented afterload-induced Ca2(+) sparks. This differential effect may arise from localized NO signaling, arising from the proximity of nNOS to RyR, as determined by super-resolution imaging. Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) also contributed to afterload-induced Ca(2+) sparks. Cardiomyocytes from a mouse model of familial hypertrophic cardiomyopathy exhibited enhanced mechanotransduction and frequent arrhythmogenic Ca(2+) sparks. Inhibiting nNOS and CaMKII, but not NOX2, in cardiomyocytes from this model eliminated the Ca2(+) sparks, suggesting mechanotransduction activated nNOS and CaMKII independently from NOX2. Thus, our data identify nNOS, CaMKII, and NOX2 as key mediators in mechanochemotransduction during cardiac contraction, which provides new therapeutic targets for treating mechanical stress-induced Ca(2+) dysregulation, arrhythmias, and cardiomyopathy.

CaMKII activity in the ventral tegmental area gates cocaine-induced synaptic plasticity in the nucleus accumbens

Addictive drugs such as cocaine induce synaptic plasticity in discrete regions of the reward circuit. The aim of the present study is to investigate whether cocaine-evoked synaptic plasticity in the ventral tegmental area (VTA) and nucleus accumbens (NAc) is causally linked. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a central regulator of long-term synaptic plasticity, learning, and drug addiction. We examined whether blocking CaMKII activity in the VTA affected cocaine conditioned place preference (CPP) and cocaine-evoked synaptic plasticity in its target brain region, the NAc. TatCN21 is a CaMKII inhibitory peptide that blocks both stimulated and autonomous CaMKII activity with high selectivity. We report that intra-VTA microinjections of tatCN21 before cocaine conditioning blocked the acquisition of cocaine CPP, whereas intra-VTA microinjections of tatCN21 before saline conditioning did not significantly affect cocaine CPP, suggesting that the CaMKII inhibitor blocks cocaine CPP through selective disruption of cocaine-cue-associated learning. Intra-VTA tatCN21 before cocaine conditioning blocked cocaine-evoked depression of excitatory synaptic transmission in the shell of the NAc slices ex vivo. In contrast, intra-VTA microinjection of tatCN21 just before the CPP test did not affect the expression of cocaine CPP and cocaine-induced synaptic plasticity in the NAc shell. These results suggest that CaMKII activity in the VTA governs cocaine-evoked synaptic plasticity in the NAc during the time window of cocaine conditioning.

Activation-triggered subunit exchange between CaMKII holoenzymes facilitates the spread of kinase activity

The activation of the dodecameric Ca(2+)/calmodulin dependent kinase II (CaMKII) holoenzyme is critical for memory formation. We now report that CaMKII has a remarkable property, which is that activation of the holoenzyme triggers the exchange of subunits between holoenzymes, including unactivated ones, enabling the calcium-independent phosphorylation of new subunits. We show, using a single-molecule TIRF microscopy technique, that the exchange process is triggered by the activation of CaMKII, and that exchange is modulated by phosphorylation of two residues in the calmodulin-binding segment, Thr 305 and Thr 306. Based on these results, and on the analysis of molecular dynamics simulations, we suggest that the phosphorylated regulatory segment of CaMKII interacts with the central hub of the holoenzyme and weakens its integrity, thereby promoting exchange. Our results have implications for an earlier idea that subunit exchange in CaMKII may have relevance for information storage resulting from brief coincident stimuli during neuronal signaling. DOI: http://dx.doi.org/10.7554/eLife.01610.001.

Cardioprotective 3',4'-dihydroxyflavonol attenuation of JNK and p38(MAPK) signalling involves CaMKII inhibition

DiOHF (3',4'-dihydroxyflavonol) is cardioprotective against I/R (ischaemia/reperfusion) injury. The biological activities of flavonols are associated with kinase modulation to alter cell signalling. We thus investigated the effects of DiOHF on the activation of MAPKs (mitogen-activated protein kinases) that regulate the cardiac stress response. In an ovine model of I/R, JNK (c-Jun N-terminal kinase), p38(MAPK), ERK (extracellular-signal-regulated kinase) and Akt were activated, and NP202, a pro-drug of DiOHF, reduced infarct size and inhibited JNK and p38(MAPK) activation, whereas ERK and Akt phosphorylation were unaltered. Similarly, in cultured myoblasts, DiOHF pre-treatment preserved viability and inhibited activation of JNK and p38(MAPK), but not ERK in response to acute oxidative and chemotoxic stress. Furthermore, DiOHF prevented stress-activation of the direct upstream regulators MKK4/7 (MAPK kinase 4/7) and MKK3/6 respectively. We utilized small-molecule affinity purification and identified CaMKII (Ca(2+)/calmodulin-dependent protein kinase II) as a kinase targeted by DiOHF and demonstrated potent CaMKII inhibition by DiOHF in vitro. Moreover, the specific inhibition of CaMKII with KN-93, but not KN-92, prevented oxidative stress-induced activation of JNK and p38(MAPK). The present study indicates DiOHF inhibition of CaMKII and attenuation of MKK3/6→p38(MAPK) and MKK4/7→JNK signalling as a requirement for the protective effects of DiOHF against stress stimuli and myocardial I/R injury.

Enzymatic activity of CaMKII is not required for its interaction with the glutamate receptor subunit GluN2B

Binding of the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) to the NMDA-type glutamate receptor subunit GluN2B is an important control mechanism for the regulation of synaptic strength. CaMKII binding to GluN2B and CaMKII translocation to synapses are induced by an initial Ca²⁺/CaM stimulus, which also activates the kinase. Indeed, several mechanistically different CaMKII inhibitors [tatCN21 and KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphonamide)] and inactivating mutations (K42M, A302R, and T305/T306D) impair this interaction, suggesting that it requires CaMKII enzymatic activity. However, this study shows that two general kinase inhibitors, H7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine] and staurosporine (Sta), which inhibit CaMKII activity by yet another mechanism, did not interfere with GluN2B binding in vitro or within cells. In contrast to a previous report, we found that Sta, like H7, inhibited CaMKII in an ATP-competitive manner. Nucleotide binding significantly enhances CaMKII/GluN2B binding in vitro, but the nucleotide competition by H7 or Sta did not prevent this effect and instead even mimicked it. H7 (700 µM) and Sta (2 µM) efficiently blocked enzymatic activity of CaMKII, both in vitro and within cells. However, neither H7 nor Sta prevented Ca²⁺-induced translocation of CaMKII to GluN2B in heterologous cells or to synapses in hippocampal neurons. Thus, activity of CaMKII (or of any other kinase inhibited by H7 or Sta) is not required for stimulation-induced GluN2B-binding or synaptic translocation of CaMKII, despite previous indication to the contrary. This shows that results with inhibitors and inhibiting mutants can be caused by structural effects independent from catalytic activity, and that detailed understanding of the mechanisms is required for their interpretation.

Loss of calcium/calmodulin-dependent protein kinase II activity in cortical astrocytes decreases glutamate uptake and induces neurotoxic release of ATP

The extent of calcium/calmodulin-dependent protein kinase II (CaMKII) inactivation in the brain after ischemia correlates with the extent of damage. We have previously shown that a loss of CaMKII activity in neurons is detrimental to neuronal viability by inducing excitotoxic glutamate release. In the current study we extend these findings to show that the ability of astrocytes to buffer extracellular glutamate is reduced when CaMKII is inhibited. Furthermore, CaMKII inhibition in astrocytes is associated with the rapid onset of intracellular calcium oscillations. Surprisingly, this rapid calcium influx is blocked by the N-type calcium channel antagonist, ω-conotoxin. Although the function of N-type calcium channels within astrocytes is controversial, these voltage-gated calcium channels have been linked to calcium-dependent vesicular gliotransmitter release. When extracellular glutamate and ATP levels are measured after CaMKII inhibition within our enriched astrocyte cultures, no alterations in glutamate levels are observed, whereas ATP levels in the extracellular environment significantly increase. Extracellular ATP accumulation associated with CaMKII inhibition contributes both to calcium oscillations within astrocytes and ultimately cortical neuron toxicity. Thus, a loss of CaMKII signaling within astrocytes dysregulates glutamate uptake and supports ATP release, two processes that would compromise neuronal survival after ischemic/excitotoxic insults.

Studying signal transduction in single dendritic spines

Many forms of synaptic plasticity are triggered by biochemical signaling that occurs in small postsynaptic compartments called dendritic spines, each of which typically houses the postsynaptic terminal associated with a single glutamatergic synapse. Recent advances in optical techniques allow investigators to monitor biochemical signaling in single dendritic spines and thus reveal the signaling mechanisms that link synaptic activity and the induction of synaptic plasticity. This is mostly in the study of Ca2+-dependent forms of synaptic plasticity for which many of the steps between Ca2+ influx and changes to the synapse are now known. This article introduces the new techniques used to investigate signaling in single dendritic spines and the neurobiological insights that they have produced.

Calmodulin-dependent protein kinase II: linking heart failure and arrhythmias

Understanding relationships between heart failure and arrhythmias, important causes of suffering and sudden death, remains an unmet goal for biomedical researchers and physicians. Evidence assembled over the past decade supports a view that activation of the multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) favors myocardial dysfunction and cell membrane electrical instability. CaMKII activation follows increases in intracellular Ca(2+) or oxidation, upstream signals with the capacity to transition CaMKII into a Ca(2+) and calmodulin-independent constitutively active enzyme. Constitutively active CaMKII appears poised to participate in disease pathways by catalyzing the phosphorylation of classes of protein targets important for excitation-contraction coupling and cell survival, including ion channels and Ca(2+) homeostatic proteins, and transcription factors that drive hypertrophic and inflammatory gene expression. This rich diversity of downstream targets helps to explain the potential for CaMKII to simultaneously affect mechanical and electrical properties of heart muscle cells. Proof-of-concept studies from a growing number of investigators show that CaMKII inhibition is beneficial for improving myocardial performance and for reducing arrhythmias. We review the molecular physiology of CaMKII and discuss CaMKII actions at key cellular targets and results of animal models of myocardial hypertrophy, dysfunction, and arrhythmias that suggest CaMKII inhibition may benefit myocardial function while reducing arrhythmias.

A significant but rather mild contribution of T286 autophosphorylation to Ca2+/CaM-stimulated CaMKII activity

Background

Autophosphorylation of the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) at T286 generates partially Ca²⁺/CaM-independent “autonomous” activity, which is thought to be required for long-term potentiation (LTP), a form of synaptic plasticity thought to underlie learning and memory. A requirement for T286 autophosphorylation also for efficient Ca²⁺/CaM-stimulated CaMKII activity has been described, but remains controversial.

Methodology/Principal Findings

In order to determine the contribution of T286 autophosphorylation to Ca²⁺/CaM-stimulated CaMKII activity, the activity of CaMKII wild type and its phosphorylation-incompetent T286A mutant was compared. As the absolute activity can vary between individual kinase preparations, the activity was measured in six different extracts for each kinase (expressed in HEK-293 cells). Consistent with measurements on purified kinase (from a baculovirus/Sf9 cell expression system), CaMKII T286A showed a mildly but significantly reduced rate of Ca²⁺/CaM-stimulated phosphorylation for two different peptide substrates (to ∼75–84% of wild type). Additional slower CaMKII autophosphorylation at T305/306 inhibits stimulation by Ca²⁺/CaM, but occurs only minimally for CaMKII wild type during CaM-stimulated activity assays. Thus, we tested if the T286A mutant may show more extensive inhibitory autophosphorylation, which could explain its reduced stimulated activity. By contrast, inhibitory autophosphorylation was instead found to be even further reduced for the T286A mutant under our assay conditions. On a side note, the phospho-T305 antibody showed some basal background immuno-reactivity also with non-phosphorylated CaMKII, as indicated by T305/306A mutants.

Conclusions/Significance

These results indicate that Ca²⁺/CaM-stimulated CaMKII activity is mildly (∼1.2–1.3fold) further increased by additional T286 autophosphorylation, but that this autophosphorylation is not required for the major part of the stimulated activity. This indicates that the phenotype of CaMKII T286A mutant mice is indeed due to the lack of autonomous activity, as the T286A mutant showed no dramatic reduction in stimulated activity.

Propagation of CaMKII translocation waves in heterogeneous spiny dendrites

CaMKII (Ca²⁺-calmodulin-dependent protein kinase II) is a key regulator of glutamatergic synapses and plays an essential role in many forms of synaptic plasticity. It has recently been observed experimentally that stimulating a local region of dendrite not only induces the local translocation of CaMKII from the dendritic shaft to synaptic targets within spines, but also initiates a wave of CaMKII translocation that spreads distally through the dendrite with an average speed of order 1 μm/s. We have previously developed a simple reaction-diffusion model of CaMKII translocation waves that can account for the observed wavespeed and predicts wave propagation failure if the density of spines is too high. A major simplification of our previous model was to treat the distribution of spines as spatially uniform. However, there are at least two sources of heterogeneity in the spine distribution that occur on two different spatial scales. First, spines are discrete entities that are joined to a dendritic branch via a thin spine neck of submicron radius, resulting in spatial variations in spine density at the micron level. The second source of heterogeneity occurs on a much longer length scale and reflects the experimental observation that there is a slow proximal to distal variation in the density of spines. In this paper, we analyze how both sources of heterogeneity modulate the speed of CaMKII translocation waves along a spiny dendrite. We adapt methods from the study of the spread of biological invasions in heterogeneous environments, including homogenization theory of pulsating fronts and Hamilton-Jacobi dynamics of sharp interfaces.

CaMKII is differentially localized in synaptic regions of Kenyon cells within the mushroom bodies of the honeybee brain

Calcium/calmodulin-dependent protein kinase II (CaMKII) has been linked to neuronal plasticity associated with long-term potentiation as well as structural synaptic plasticity. Previous work in adult honeybees has shown that a single CaMKII gene is strongly expressed in the mushroom bodies (MBs), brain centers associated with sensory integration, and learning and memory formation. To study a potential role of CaMKII in synaptic plasticity, the cellular and subcellular distribution of activated (phosphorylated) pCaMKII protein was investigated at various life stages of the honeybee using immunocytochemistry, confocal microscopy, and western blot analyses. Whereas at pupal stages 3-4 most parts of the brain showed high levels of pCaMKII immunoreactivity, the protein was predominantly concentrated in the MBs in the adult brain. The results show that pCaMKII is present in a specific subpopulation of Kenyon cells, the noncompact cells. Within the olfactory (lip) and visual (collar) subregion of the MB calyx neuropil pCaMKII was colocalized with f-actin in postsynaptic compartments of microglomeruli, indicating that it is enriched in Kenyon cell dendritic spines. This suggests a potential role of CaMKII in Kenyon cell dendritic plasticity. Interestingly, pCaMKII protein was absent in two other types of Kenyon cells, the inner compact cells associated with the multimodal basal ring and the outer compact cells. During adult behavioral maturation from nurse bees to foragers, pCaMKII distribution remained essentially similar at the qualitative level, suggesting a potential role in dendritic plasticity of Kenyon cells throughout the entire life span of a worker bee.

Improving a natural CaMKII inhibitor by random and rational design

Background

CaM-KIIN has evolved to inhibit stimulated and autonomous activity of the Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) efficiently, selectively, and potently (IC50 ∼100 nM). The CN class of peptides, derived from the inhibitory region of CaM-KIIN, provides powerful new tools to study CaMKII functions. The goal of this study was to identify the residues required for CaMKII inhibition, and to assess if artificial mutations could further improve the potency achieved during evolution.

Methodology/Principal Findings

First, the minimal region with full inhibitory potency was identified (CN19) by determining the effect of truncated peptides on CaMKII activity in biochemical assays. Then, individual residues of CN19 were mutated. Most individual Ala substitutions decreased potency of CaMKII inhibition, however, P3A, K13A, and R14A increased potency. Importantly, this initial Ala scan suggested a specific interaction of the region around R11 with the CaMKII substrate binding site, which was exploited for further rational mutagenesis to generate an optimized pseudo-substrate sequence. Indeed, the potency of the optimized peptide CN19o was >250fold improved (IC50 <0.4 nM), and CN19o has characteristics of a tight-binding inhibitor. The selectivity for CaMKII versus CaMKI was similarly improved (to almost 100,000fold for CN19o). A phospho-mimetic S12D mutation decreased potency, indicating potential for regulation by cellular signaling. Consistent with importance of this residue in inhibition, most other S12 mutations also significantly decreased potency, however, mutation to V or Q did not.

Conlusions/Significance

These results provide improved research tools for studying CaMKII function, and indicate that evolution fine-tuned CaM-KIIN not for maximal potency of CaMKII inhibition, but for lower potency that may be optimal for dynamic regulation of signal transduction.

Nucleotides and phosphorylation bi-directionally modulate Ca2+/calmodulin-dependent protein kinase II (CaMKII) binding to the N-methyl-D-aspartate (NMDA) receptor subunit GluN2B

The Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) and the NMDA-type glutamate receptor are key regulators of synaptic plasticity underlying learning and memory. Direct binding of CaMKII to the NMDA receptor subunit GluN2B (formerly known as NR2B) (i) is induced by Ca(2+)/CaM but outlasts this initial Ca(2+)-stimulus, (ii) mediates CaMKII translocation to synapses, and (iii) regulates synaptic strength. CaMKII binds to GluN2B around S1303, the major CaMKII phosphorylation site on GluN2B. We show here that a phospho-mimetic S1303D mutation inhibited CaM-induced CaMKII binding to GluN2B in vitro, presenting a conundrum how binding can occur within cells, where high ATP concentration should promote S1303 phosphorylation. Surprisingly, addition of ATP actually enhanced the binding. Mutational analysis revealed that this positive net effect was caused by four modulatory effects of ATP, two positive (direct nucleotide binding and CaMKII T286 autophosphorylation) and two negative (GluN2B S1303 phosphorylation and CaMKII T305/6 autophosphorylation). Imaging showed positive regulation by nucleotide binding also within transfected HEK cells and neurons. In fact, nucleotide binding was a requirement for efficient CaMKII interaction with GluN2B in cells, while T286 autophosphorylation was not. Kinetic considerations support a model in which positive regulation by nucleotide binding and T286 autophosphorylation occurs faster than negative modulation by GluN2B S1303 and CaMKII T305/6 phosphorylation, allowing efficient CaMKII binding to GluN2B despite the inhibitory effects of the two slower reactions.

CaMKII in cerebral ischemia

Ischemic insults on neurons trigger excessive, pathological glutamate release that causes Ca²⁺ overload resulting in neuronal cell death (excitotoxicity). The Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of physiological excitatory glutamate signals underlying neuronal plasticity and learning. Glutamate stimuli trigger autophosphorylation of CaMKII at T286, a process that makes the kinase "autonomous" (partially active independent from Ca²⁺ stimulation) and that is required for forms of synaptic plasticity. Recent studies suggested autonomous CaMKII activity also as potential drug target for post-insult neuroprotection, both after glutamate insults in neuronal cultures and after focal cerebral ischemia in vivo. However, CaMKII and other members of the CaM kinase family have been implicated in regulation of both neuronal death and survival. Here, we discuss past findings and possible mechanisms of CaM kinase functions in excitotoxicity and cerebral ischemia, with a focus on CaMKII and its regulation.

Age-related regulation of excitation-contraction coupling in rat heart

Hearts from subjects with different ages have different Ca(2+) signaling. Release of Ca(2+) from intracellular stores in response to an action potential initiates cardiac contraction. Both depolarization-stimulated and spontaneous Ca(2+) releases, Ca(2+) transients and Ca(2+) sparks, demonstrate the main events of excitation-contraction coupling (ECC). Global increase in free Ca(2+) concentration ([Ca(2+)]( i )) consists of summation of Ca(2+) release events in cardiomyocytes. Since the Ca(2+) flux induced by Ca(2+) sparks reports a summation of ryanodine-sensitive Ca(2+) release channels (RyR2s)'s behavior in a spark cluster, evaluation of the properties of Ca(2+) sparks and Ca(2+) transients may provide insight into the role of RyR2s on altered heart function between 3-month-old (young adult) and 6-month-old (mature adult) rats. Basal [Ca(2+)]( i ) and Ca(2+) sparks frequency were significantly higher in mature adult rats compared to those of young adults. Moreover, amplitudes of Ca(2+) sparks and Ca(2+) transients were significantly smaller in mature adults than those of young adults with longer time courses. A smaller L-type Ca(2+) current density and decreased SR Ca(2+) load was observed in mature adult rats. In addition, RyR2s were markedly hyperphosphorylated, and phosphorylation levels of PKA and CaMKII were higher in heart from mature adults compared to those of young adults, whereas their SERCA protein levels were similar. Our data demonstrate that hearts from rats with different ages have different Ca(2+) signaling including hyperphosphorylation of RyR2s and higher basal [Ca(2+)]( i ) together with increased oxidized protein-thiols in mature adult rats compared to those of young adults, which play important roles in ECC. Finally, we report that ECC efficiency changes with age during maturation, partially related with an increased cellular oxidation level leading to reduced free protein-thiols in cardiomyocytes.

LTP in hippocampal neurons is associated with a CaMKII-mediated increase in GluA1 surface expression

The use of hippocampal dissociated neuronal cultures has enabled the study of molecular changes in endogenous native proteins associated with long-term potentiation. Using immunofluorescence labelling of the active (Thr286-phosphorylated) alpha-Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) we found that CaMKII activity was increased by transient (3 × 1 s) depolarisation in 18- to 21-day-old cultures but not in 9- to 11-day-old cultures. The increase in Thr286 phosphorylation of CaMKII required the activation of NMDA receptors and was greatly attenuated by the CaMKII inhibitor KN-62. We compared the effects of transient depolarisation on the surface expression of GluA1 and GluA2 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor and found a preferential recruitment of the GluA1 subunit. CaMKII inhibition prevented this NMDA receptor-dependent delivery of GluA1 to the cell surface. CaMKII activation is therefore an important factor in the activity-dependent recruitment of native GluA1 subunit-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors to the cell surface of hippocampal neurons.

CaMKII "autonomy" is required for initiating but not for maintaining neuronal long-term information storage

Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) "autonomy" (T286-autophosphorylation-induced Ca(2+)-independent activity) is required for long-term potentiation (LTP) and for learning and memory, as demonstrated by CaMKII T286A mutant mice. The >20-year-old hypothesis that CaMKII stimulation is required for LTP induction, while CaMKII autonomy is required for LTP maintenance was recently supported using the cell-penetrating fusion-peptide inhibitor antCN27. However, we demonstrate here that ant/penetratin fusion to CN27 compromised CaMKII-selectivity, by enhancing a previously unnoticed direct binding of CaM to ant/penetratin. In contrast to antCN27, the improved cell-penetrating inhibitor tatCN21 (5 mum) showed neither CaM binding nor inhibition of basal synaptic transmission. In vitro, tatCN21 inhibited stimulated and autonomous CaMKII activity with equal potency. In rat hippocampal slices, tatCN21 inhibited LTP induction, but not LTP maintenance. Correspondingly, tatCN21 also inhibited learning, but not memory storage or retrieval in a mouse in vivo model. Thus, CaMKII autonomy provides a short-term molecular memory that is important in the signal computation leading to memory formation, but is not required as long-term memory store.

Identification and inhibitory properties of a novel Ca(2+)/calmodulin antagonist

We developed a high-throughput yeast-based assay to screen for chemical inhibitors of Ca(2+)/calmodulin-dependent kinase pathways. After screening two small libraries, we identified the novel antagonist 125-C9, a substituted ethyleneamine. In vitro kinase assays confirmed that 125-C9 inhibited several calmodulin-dependent kinases (CaMKs) competitively with Ca(2+)/calmodulin (Ca(2+)/CaM). This suggested that 125-C9 acted as an antagonist for Ca(2+)/CaM rather than for CaMKs. We confirmed this hypothesis by showing that 125-C9 binds directly to Ca(2+)/CaM using isothermal titration calorimetry. We further characterized binding of 125-C9 to Ca(2+)/CaM and compared its properties with those of two well-studied CaM antagonists: trifluoperazine (TFP) and W-13. Isothermal titration calorimetry revealed that binding of 125-C9 to CaM is absolutely Ca(2+)-dependent, likely occurs with a stoichiometry of five 125-C9 molecules to one CaM molecule, and involves an exchange of two protons at pH 7.0. Binding of 125-C9 is driven overall by entropy and appears to be competitive with TFP and W-13, which is consistent with occupation of similar binding sites. To test the effects of 125-C9 in living cells, we evaluated mitogen-stimulated re-entry of quiescent cells into proliferation and found similar, although slightly better, levels of inhibition by 125-C9 than by TFP and W-13. Our results not only define a novel Ca(2+)/CaM inhibitor but also reveal that chemically unique CaM antagonists can bind CaM by distinct mechanisms but similarly inhibit cellular actions of CaM.

CaMKII autonomy is substrate-dependent and further stimulated by Ca2+/calmodulin

A hallmark feature of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) regulation is the generation of Ca(2+)-independent autonomous activity by Thr-286 autophosphorylation. CaMKII autonomy has been regarded a form of molecular memory and is indeed important in neuronal plasticity and learning/memory. Thr-286-phosphorylated CaMKII is thought to be essentially fully active ( approximately 70-100%), implicating that it is no longer regulated and that its dramatically increased Ca(2+)/CaM affinity is of minor functional importance. However, this study shows that autonomy greater than 15-25% was the exception, not the rule, and required a special mechanism (T-site binding; by the T-substrates AC2 or NR2B). Autonomous activity toward regular R-substrates (including tyrosine hydroxylase and GluR1) was significantly further stimulated by Ca(2+)/CaM, both in vitro and within cells. Altered K(m) and V(max) made autonomy also substrate- (and ATP) concentration-dependent, but only over a narrow range, with remarkable stability at physiological concentrations. Such regulation still allows molecular memory of previous Ca(2+) signals, but prevents complete uncoupling from subsequent cellular stimulation.